1. Field of the Invention
The present invention relates to a cathode ray tube, more particularly, to a cathode ray tube including a shadow mask having an improved drop characteristic by adjusting a curvature thereof.
2. Discussion of the Background Art
FIG. 1 illustrates the structure of a related art color cathode ray tube.
As depicted in FIG. 1, a panel 1 and a funnel 2 of the color cathode ray tube are sealed up (or connected) tightly together, so the inside of the cathode ray tube is generally in a vacuum state.
To see the structure of the cathode ray tube, a fluorescent screen 3 with red (R), green (G) and blue (B) primary color phosphors (or fluorescent substances) is formed inside of the panel 1, and an electron gun 4 for emitting three color electron beams 7, namely red, green and blue, is housed in the neck portion of the funnel on the opposite side of the fluorescent screen 3.
A shadow mask 5 having a color selecting function is disposed at a predetermined space between the fluorescent screen 3 and the electron gun 4, more specifically, closer to the fluorescent screen 3. Also, in order to restrict the motion of the electron beams 7 promoted by a magnetic field, an inner shield 6, which is made of magnetic substance, is provided to a rear side of the cathode ray tube to diminish an influence of a magnetic field thereon.
Meanwhile, there is a convergence purity correcting magnet (CPM) 8 in the neck portion of the funnel 2, which serves to adjust R, G and B electron beams emitted from the electron gun 4 to be converged to one single point, and in front of the magnet 8, there is a deflection yoke 9 for deflecting the electron beams 7.
In addition, a reinforcing band 10 is put on the external skirt area of the panel so as to reinforce a front surface glass with the presence of a high internal vacuum state. In other words, since the cathode ray tube is highly evacuated, it can be easily exploded by external impacts. To obviate this problem, the panel is specially designed to be able to sustain atmospheric pressure. As aforementioned, the reinforcing band 10 is clamped to the external skirt area of the panel 1, dispersing stress upon the highly evacuated cathode ray tube and thereby, making the panel resistant to external impacts.
To briefly explain how the color cathode ray tube with the above construction operates, the electron beams 7 emitted from the electron gun 4 are deflected in the horizontal and vertical directions according to the deflection yoke 9, and the deflected electron beams 7 pass through a beam passing hole on the shadow mask 5 and eventually strike the fluorescent screen 3 on the front side, thereby displaying a desired color image.
FIG. 2 illustrates a related art shadow mask and mask spring.
Referring to FIG. 2, the shadow mask 5 is attached to a mask frame 11, and the mask frame 11 is coupled onto an inner surface of a panel 1 by a mask spring 12. Although the shadow mask 5 in the drawing is welded to a welding portion 15 of the inner surface of the mask frame 11, it can also be welded to an outer surface of the mask frame 11.
Electron beam passing holes formed on the shadow mask 5 select colors of electron beams, and when the electron beams strike a front surface of a fluorescent screen 3, a desired image is displayed on the screen.
As depicted in FIG. 2, X-axis is a major-axis direction, Y-axis is a minor-axis direction, and D-axis is a diagonal-axis direction. In each direction, a different curvature is fixed, and thus has a different impact resistance from external impacts.
Also, Z-axis is a perpendicular direction from a center portion of the shadow mask.
When external impacts are given to the shadow mask 5, the center portion is sometimes recessed (dropped) or peripheral portion is sometimes distorted.
To improve a drop characteristic against external impacts, manufacturers have used different materials for the shadow mask 5, or changed a welding position, or formed a plurality of embossments thereon.
Particularly, external impacts on the shadow mask are biggest in the normal direction of the curved surface. As shown in FIG. 3, since the major-axis direction curvature the shadow mask 5 is great, external impacts F are not applied directly or fully in that direction, and thus the shadow mask is not severely distorted.
On the other hand, the minor-axis direction curvature of the shadow mask 5 is not large. Thus, external impacts F are almost perpendicularly applied in that direction, and as a result, the shadow mask 5 is very severely distorted.
FIG. 5 graphically illustrates a relation between external impacts F and distortion amounts of the shadow mask 5.
As shown on the graph, as an external impacts F is increased, the amount of distortion of the shadow mask 5 also increases proportionally, and at A point, it is no longer increased in proportion to the external impacts F. However, after B point, the amount of distortion of the shadow mask 5 is again increased in proportion to the external impact F.
Between A point and B point, a buckling phenomenon occurs, so even if the external impact F is absent, the shadow mask 5 does not return to its original shape.
That is to say, when the curvature of the shadow mask 5 is distorted by the external impact F, the shadow mask cannot select colors of electron beams more effectively, and this causes deteriorations in picture quality of a cathode ray tube.
Many attempts have been made to solve the above problem. One of them is changing material and thickness of the shadow mask 5 to reinforce drop characteristics of the shadow mask 5.
For example, a material having a high Young's modulus value was used or the thickness of the shadow mask was increased in order to strengthen the drop characteristics.
However, these traditional methods only increased price of the shadow mask 5.
As an alternative, manufacturers tried to make the curved surface of the shadow mask 5 close to a welding point to which the shadow mask 5 and the mask frame 11 are welded by increasing the height of the welding point. However, this method also gave rise to a side effect that the shadow mask 5 and the mask frame 11 were thermally expanded severely, resultantly deteriorating a doming characteristic of the shadow mask 5.
Other manufacturers suggested forming a plurality of embossments on the shadow mask 5. Unfortunately however, the effect thereof was not significant, and it only made it difficult to form a curved surface for the shadow mask 5.
FIG. 6 shows different radius of curvature of a related art shadow mask.
More specifically, the graph in FIG. 6 illustrates the relation between a radius of curvature of the shadow mask and a distance from the shadow mask center in a major-axis, minor-axis and diagonal-axis direction, respectively.
As shown on the graph, the radius of curvature is largest from the center of the shadow mask to the minor-axis direction, and gradually reduces in order of the diagonal-axis and major-axis directions.
That is to say, in case of the related art shadow mask, the radius of curvature in the minor-axis direction, Ry, the radius of curvature in the diagonal-axis direction, Rd, and the radius of curvature in the major-axis direction, Rx, satisfy a relation of Rx<Rd<Ry. This relation is maintained not only at the central portion of the shadow mask but also in the peripheral portion of the shadow mask.
Here, a large radius of curvature means that the surface is flat. Therefore, as discussed before with reference of FIG. 3, the shadow mask is relatively more flat and thus weaker in the minor-axis direction than in the major-axis or diagonal-axis directions, experiencing more of external impacts.
In short, the related art shadow mask posed a problem that its strength in the minor-axis direction is relatively weak, eventually influencing on the overall quality of the shadow mask and deteriorating a picture quality of the cathode ray tube.